Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)

3. Designing Application Protocols
This section provides guidelines for designers of application
protocols, in the form of a checklist to follow when reusing the
recommendations provided in this document.
o Does your technology use DNS SRV records to resolve the DNS domain
names of application services? If so, consider recommending or
requiring support for the SRV-ID identifier type in PKIX
certificates issued and used in your technology community. (Note
that many existing application technologies use DNS SRV records to
resolve the DNS domain names of application services, but do not
rely on representations of those records in PKIX certificates by
means of SRV-IDs as defined in [SRVNAME].)
o Does your technology use URIs to identify application services?
If so, consider recommending or requiring support for the URI-ID
identifier type. (Note that many existing application
technologies use URIs to identify application services, but do not
rely on representation of those URIs in PKIX certificates by means
of URI-IDs.)
o Does your technology need to use DNS domain names in the Common
Name of certificates for the sake of backward compatibility? If
so, consider recommending support for the CN-ID identifier type as
a fallback.
o Does your technology need to allow the wildcard character in DNS
domain names? If so, consider recommending support for wildcard
certificates, and specify exactly where the wildcard character is
allowed to occur (e.g., only the complete left-most label of a DNS
domain name).
Sample text is provided under Appendix A.
4. Representing Server Identity
This section provides rules and guidelines for issuers of
certificates.
4.1. Rules
When a certification authority issues a certificate based on the
fully qualified DNS domain name at which the application service
provider will provide the relevant application, the following rules
apply to the representation of application service identities. The

reader needs to be aware that some of these rules are cumulative and
can interact in important ways that are illustrated later in this
document.
1. The certificate SHOULD include a "DNS-ID" if possible as a
baseline for interoperability.
2. If the service using the certificate deploys a technology for
which the relevant specification stipulates that certificates
ought to include identifiers of type SRV-ID (e.g., this is true
of [XMPP]), then the certificate SHOULD include an SRV-ID.
3. If the service using the certificate deploys a technology for
which the relevant specification stipulates that certificates
ought to include identifiers of type URI-ID (e.g., this is true
of [SIP] as specified by [SIP-CERTS], but not true of [HTTP]
since [HTTP-TLS] does not describe usage of a URI-ID for HTTP
services), then the certificate SHOULD include a URI-ID. The
scheme SHALL be that of the protocol associated with the
application service type and the "host" component (or its
equivalent) SHALL be the fully qualified DNS domain name of the
service. A specification that reuses this one MUST specify which
URI schemes are to be considered acceptable in URI-IDs contained
in PKIX certificates used for the application protocol (e.g.,
"sip" but not "sips" or "tel" for SIP as described in [SIP-SIPS],
or perhaps http and https for HTTP as might be described in a
future specification).
4. The certificate MAY include other application-specific
identifiers for types that were defined before publication of
[SRVNAME] (e.g., XmppAddr for [XMPP]) or for which service names
or URI schemes do not exist; however, such application-specific
identifiers are not applicable to all application technologies
and therefore are out of scope for this specification.
5. Even though many deployed clients still check for the CN-ID
within the certificate subject field, certification authorities
are encouraged to migrate away from issuing certificates that
represent the server's fully qualified DNS domain name in a
CN-ID. Therefore, the certificate SHOULD NOT include a CN-ID
unless the certification authority issues the certificate in
accordance with a specification that reuses this one and that
explicitly encourages continued support for the CN-ID identifier
type in the context of a given application technology.
6. The certificate MAY contain more than one DNS-ID, SRV-ID, or
URI-ID but SHOULD NOT contain more than one CN-ID, as further
explained under Section 7.4.

7. Unless a specification that reuses this one allows continued
support for the wildcard character '*', the DNS domain name
portion of a presented identifier SHOULD NOT contain the wildcard
character, whether as the complete left-most label within the
identifier (following the description of labels and domain names
in [DNS-CONCEPTS], e.g., "*.example.com") or as a fragment
thereof (e.g., *oo.example.com, f*o.example.com, or
fo*.example.com). A more detailed discussion of so-called
"wildcard certificates" is provided under Section 7.2.
4.2. Examples
Consider a simple website at "www.example.com", which is not
discoverable via DNS SRV lookups. Because HTTP does not specify the
use of URIs in server certificates, a certificate for this service
might include only a DNS-ID of "www.example.com". It might also
include a CN-ID of "www.example.com" for backward compatibility with
deployed infrastructure.
Consider an IMAP-accessible email server at the host
"mail.example.net" servicing email addresses of the form
"user@example.net" and discoverable via DNS SRV lookups on the
application service name of "example.net". A certificate for this
service might include SRV-IDs of "_imap.example.net" and
"_imaps.example.net" (see [EMAIL-SRV]) along with DNS-IDs of
"example.net" and "mail.example.net". It might also include CN-IDs
of "example.net" and "mail.example.net" for backward compatibility
with deployed infrastructure.
Consider a SIP-accessible voice-over-IP (VoIP) server at the host
"voice.example.edu" servicing SIP addresses of the form
"user@voice.example.edu" and identified by a URI of <sip:
voice.example.edu>. A certificate for this service would include a
URI-ID of "sip:voice.example.edu" (see [SIP-CERTS]) along with a
DNS-ID of "voice.example.edu". It might also include a CN-ID of
"voice.example.edu" for backward compatibility with deployed
infrastructure.
Consider an XMPP-compatible instant messaging (IM) server at the host
"im.example.org" servicing IM addresses of the form
"user@im.example.org" and discoverable via DNS SRV lookups on the
"im.example.org" domain. A certificate for this service might
include SRV-IDs of "_xmpp-client.im.example.org" and
"_xmpp-server.im.example.org" (see [XMPP]), a DNS-ID of
"im.example.org", and an XMPP-specific "XmppAddr" of "im.example.org"
(see [XMPP]). It might also include a CN-ID of "im.example.org" for
backward compatibility with deployed infrastructure.

5. Requesting Server Certificates
This section provides rules and guidelines for service providers
regarding the information to include in certificate signing requests
(CSRs).
In general, service providers are encouraged to request certificates
that include all of the identifier types that are required or
recommended for the application service type that will be secured
using the certificate to be issued.
If the certificate might be used for any type of application service,
then the service provider is encouraged to request a certificate that
includes only a DNS-ID.
If the certificate will be used for only a single type of application
service, then the service provider is encouraged to request a
certificate that includes a DNS-ID and, if appropriate for the
application service type, an SRV-ID or URI-ID that limits the
deployment scope of the certificate to only the defined application
service type.
If a service provider offering multiple application service types
(e.g., a World Wide Web service, an email service, and an instant
messaging service) wishes to limit the applicability of certificates
using SRV-IDs or URI-IDs, then the service provider is encouraged to
request multiple certificates, i.e., one certificate per application
service type. Conversely, the service provider is discouraged from
requesting a single certificate containing multiple SRV-IDs or URI-
IDs identifying each different application service type. This
guideline does not apply to application service type "bundles" that
are used to identify manifold distinct access methods to the same
underlying application (e.g., an email application with access
methods denoted by the application service types of "imap", "imaps",
"pop3", "pop3s", and "submission" as described in [EMAIL-SRV]).
6. Verifying Service Identity
This section provides rules and guidelines for implementers of
application client software regarding algorithms for verification of
application service identity.
6.1. Overview
At a high level, the client verifies the application service's
identity by performing the actions listed below (which are defined in
the following subsections of this document):

1. The client constructs a list of acceptable reference identifiers
based on the source domain and, optionally, the type of service
to which the client is connecting.
2. The server provides its identifiers in the form of a PKIX
certificate.
3. The client checks each of its reference identifiers against the
presented identifiers for the purpose of finding a match.
4. When checking a reference identifier against a presented
identifier, the client matches the source domain of the
identifiers and, optionally, their application service type.
Naturally, in addition to checking identifiers, a client might
complete further checks to ensure that the server is authorized to
provide the requested service. However, such checking is not a
matter of verifying the application service identity presented in a
certificate, and therefore methods for doing so (e.g., consulting
local policy information) are out of scope for this document.
6.2. Constructing a List of Reference Identifiers
6.2.1. Rules
The client MUST construct a list of acceptable reference identifiers,
and MUST do so independently of the identifiers presented by the
service.
The inputs used by the client to construct its list of reference
identifiers might be a URI that a user has typed into an interface
(e.g., an HTTPS URL for a website), configured account information
(e.g., the domain name of a particular host or URI used for
retrieving information or connecting to a network, which might be
different from the DNS domain name portion of a username), a
hyperlink in a web page that triggers a browser to retrieve a media
object or script, or some other combination of information that can
yield a source domain and an application service type.
The client might need to extract the source domain and application
service type from the input(s) it has received. The extracted data
MUST include only information that can be securely parsed out of the
inputs (e.g., parsing the fully qualified DNS domain name out of the
"host" component (or its equivalent) of a URI or deriving the
application service type from the scheme of a URI) or information
that is derived in a manner not subject to subversion by network
attackers (e.g., pulling the data from a delegated domain that is
explicitly established via client or system configuration, resolving

the data via [DNSSEC], or obtaining the data from a third-party
domain mapping service in which a human user has explicitly placed
trust and with which the client communicates over a connection or
association that provides both mutual authentication and integrity
checking). These considerations apply only to extraction of the
source domain from the inputs; naturally, if the inputs themselves
are invalid or corrupt (e.g., a user has clicked a link provided by a
malicious entity in a phishing attack), then the client might end up
communicating with an unexpected application service.
Example: Given an input URI of <sips:alice@example.net>, a client
would derive the application service type "sip" from the "scheme"
and parse the domain name "example.net" from the "host" component
(or its equivalent).
Each reference identifier in the list SHOULD be based on the source
domain and SHOULD NOT be based on a derived domain (e.g., a host name
or domain name discovered through DNS resolution of the source
domain). This rule is important because only a match between the
user inputs and a presented identifier enables the client to be sure
that the certificate can legitimately be used to secure the client's
communication with the server. There is only one scenario in which
it is acceptable for an interactive client to override the
recommendation in this rule and therefore communicate with a domain
name other than the source domain: because a human user has "pinned"
the application service's certificate to the alternative domain name
as further discussed under Section 6.6.4 and Section 7.1. In this
case, the inputs used by the client to construct its list of
reference identifiers might include more than one fully qualified DNS
domain name, i.e., both (a) the source domain and (b) the alternative
domain contained in the pinned certificate.
Using the combination of fully qualified DNS domain name(s) and
application service type, the client constructs a list of reference
identifiers in accordance with the following rules:
o The list SHOULD include a DNS-ID. A reference identifier of type
DNS-ID can be directly constructed from a fully qualified DNS
domain name that is (a) contained in or securely derived from the
inputs (i.e., the source domain), or (b) explicitly associated
with the source domain by means of user configuration (i.e., a
derived domain).
o If a server for the application service type is typically
discovered by means of DNS SRV records, then the list SHOULD
include an SRV-ID.

o If a server for the application service type is typically
associated with a URI for security purposes (i.e., a formal
protocol document specifies the use of URIs in server
certificates), then the list SHOULD include a URI-ID.
o The list MAY include a CN-ID, mainly for the sake of backward
compatibility with deployed infrastructure.
Which identifier types a client includes in its list of reference
identifiers is a matter of local policy. For example, in certain
deployment environments, a client that is built to connect only to a
particular kind of service (e.g., only IM services) might be
configured to accept as valid only certificates that include an
SRV-ID for that application service type; in this case, the client
would include only SRV-IDs matching the application service type in
its list of reference identifiers (not, for example, DNS-IDs). By
contrast, a more lenient client (even one built to connect only to a
particular kind of service) might include both SRV-IDs and DNS-IDs in
its list of reference identifiers.
Implementation Note: It is highly likely that implementers of
client software will need to support CN-IDs for the foreseeable
future, because certificates containing CN-IDs are so widely
deployed. Implementers are advised to monitor the state of the
art with regard to certificate issuance policies and migrate away
from support CN-IDs in the future if possible.
Implementation Note: The client does not need to construct the
foregoing identifiers in the actual formats found in a certificate
(e.g., as ASN.1 types); it only needs to construct the functional
equivalent of such identifiers for matching purposes.
Security Warning: A client MUST NOT construct a reference
identifier corresponding to Relative Distinguished Names (RDNs)
other than those of type Common Name and MUST NOT check for RDNs
other than those of type Common Name in the presented identifiers.
6.2.2. Examples
A web browser that is connecting via HTTPS to the website at
"www.example.com" might have two reference identifiers: a DNS-ID of
"www.example.com" and, as a fallback, a CN-ID of "www.example.com".
A mail user agent that is connecting via IMAPS to the email service
at "example.net" (resolved as "mail.example.net") might have five
reference identifiers: an SRV-ID of "_imaps.example.net" (see
[EMAIL-SRV]), DNS-IDs of "example.net" and "mail.example.net", and,
as a fallback, CN-IDs of "example.net" and "mail.example.net". (A

legacy email user agent would not support [EMAIL-SRV] and therefore
would probably be explicitly configured to connect to
"mail.example.net", whereas an SRV-aware user agent would derive
"example.net" from an email address of the form "user@example.net"
but might also accept "mail.example.net" as the DNS domain name
portion of reference identifiers for the service.)
A voice-over-IP (VoIP) user agent that is connecting via SIP to the
voice service at "voice.example.edu" might have only one reference
identifier: a URI-ID of "sip:voice.example.edu" (see [SIP-CERTS]).
An instant messaging (IM) client that is connecting via XMPP to the
IM service at "im.example.org" might have three reference
identifiers: an SRV-ID of "_xmpp-client.im.example.org" (see [XMPP]),
a DNS-ID of "im.example.org", and an XMPP-specific "XmppAddr" of
"im.example.org" (see [XMPP]).
6.3. Preparing to Seek a Match
Once the client has constructed its list of reference identifiers and
has received the server's presented identifiers in the form of a PKIX
certificate, the client checks its reference identifiers against the
presented identifiers for the purpose of finding a match. The search
fails if the client exhausts its list of reference identifiers
without finding a match. The search succeeds if any presented
identifier matches one of the reference identifiers, at which point
the client SHOULD stop the search.
Implementation Note: A client might be configured to perform
multiple searches, i.e., to match more than one reference
identifier. Although such behavior is not forbidden by this
specification, rules for matching multiple reference identifiers
are a matter for implementation or future specification.
Security Warning: A client MUST NOT seek a match for a reference
identifier of CN-ID if the presented identifiers include a DNS-ID,
SRV-ID, URI-ID, or any application-specific identifier types
supported by the client.
Before applying the comparison rules provided in the following
sections, the client might need to split the reference identifier
into its DNS domain name portion and its application service type
portion, as follows:
o A reference identifier of type DNS-ID does not include an
application service type portion and thus can be used directly as
the DNS domain name for comparison purposes. As an example, a

DNS-ID of "www.example.com" would result in a DNS domain name
portion of "www.example.com".
o A reference identifier of type CN-ID also does not include an
application service type portion and thus can be used directly as
the DNS domain name for comparison purposes. As previously
mentioned, this document specifies that a CN-ID always contains a
string whose form matches that of a DNS domain name (thus
differentiating a CN-ID from a Common Name containing a human-
friendly name).
o For a reference identifier of type SRV-ID, the DNS domain name
portion is the Name and the application service type portion is
the Service. As an example, an SRV-ID of "_imaps.example.net"
would be split into a DNS domain name portion of "example.net" and
an application service type portion of "imaps" (mapping to an
application protocol of IMAP as explained in [EMAIL-SRV]).
o For a reference identifier of type URI-ID, the DNS domain name
portion is the "reg-name" part of the "host" component (or its
equivalent) and the application service type portion is the
application service type associated with the scheme name matching
the [ABNF] "scheme" rule from [URI] (not including the ':'
separator). As previously mentioned, this document specifies that
a URI-ID always contains a "host" component (or its equivalent)
containing a "reg-name". (Matching only the "reg-name" rule from
[URI] limits verification to DNS domain names, thereby
differentiating a URI-ID from a uniformResourceIdentifier entry
that contains an IP address or a mere host name, or that does not
contain a "host" component at all.) Furthermore, note that
extraction of the "reg-name" might necessitate normalization of
the URI (as explained in [URI]). As an example, a URI-ID of "sip:
voice.example.edu" would be split into a DNS domain name portion
of "voice.example.edu" and an application service type of "sip"
(associated with an application protocol of SIP as explained in
[SIP-CERTS]).
Detailed comparison rules for matching the DNS domain name portion
and application service type portion of the reference identifier are
provided in the following sections.
6.4. Matching the DNS Domain Name Portion
The client MUST match the DNS domain name portion of a reference
identifier according to the following rules (and SHOULD also check
the application service type as described under Section 6.5). The
rules differ depending on whether the domain to be checked is a
"traditional domain name" or an "internationalized domain name" (as

defined under Section 2.2). Furthermore, to meet the needs of
clients that support presented identifiers containing the wildcard
character '*', we define a supplemental rule for so-called "wildcard
certificates". Finally, we also specify the circumstances under
which it is acceptable to check the "CN-ID" identifier type.
6.4.1. Checking of Traditional Domain Names
If the DNS domain name portion of a reference identifier is a
"traditional domain name", then matching of the reference identifier
against the presented identifier is performed by comparing the set of
domain name labels using a case-insensitive ASCII comparison, as
clarified by [DNS-CASE] (e.g., "WWW.Example.Com" would be lower-cased
to "www.example.com" for comparison purposes). Each label MUST match
in order for the names to be considered to match, except as
supplemented by the rule about checking of wildcard labels
(Section 6.4.3).
6.4.2. Checking of Internationalized Domain Names
If the DNS domain name portion of a reference identifier is an
internationalized domain name, then an implementation MUST convert
any U-labels [IDNA-DEFS] in the domain name to A-labels before
checking the domain name. In accordance with [IDNA-PROTO], A-labels
MUST be compared as case-insensitive ASCII. Each label MUST match in
order for the domain names to be considered to match, except as
supplemented by the rule about checking of wildcard labels
(Section 6.4.3; but see also Section 7.2 regarding wildcards in
internationalized domain names).
6.4.3. Checking of Wildcard Certificates
A client employing this specification's rules MAY match the reference
identifier against a presented identifier whose DNS domain name
portion contains the wildcard character '*' as part or all of a label
(following the description of labels and domain names in
[DNS-CONCEPTS]).
For information regarding the security characteristics of wildcard
certificates, see Section 7.2.
If a client matches the reference identifier against a presented
identifier whose DNS domain name portion contains the wildcard
character '*', the following rules apply:
1. The client SHOULD NOT attempt to match a presented identifier in
which the wildcard character comprises a label other than the
left-most label (e.g., do not match bar.*.example.net).

2. If the wildcard character is the only character of the left-most
label in the presented identifier, the client SHOULD NOT compare
against anything but the left-most label of the reference
identifier (e.g., *.example.com would match foo.example.com but
not bar.foo.example.com or example.com).
3. The client MAY match a presented identifier in which the wildcard
character is not the only character of the label (e.g.,
baz*.example.net and *baz.example.net and b*z.example.net would
be taken to match baz1.example.net and foobaz.example.net and
buzz.example.net, respectively). However, the client SHOULD NOT
attempt to match a presented identifier where the wildcard
character is embedded within an A-label or U-label [IDNA-DEFS] of
an internationalized domain name [IDNA-PROTO].
6.4.4. Checking of Common Names
As noted, a client MUST NOT seek a match for a reference identifier
of CN-ID if the presented identifiers include a DNS-ID, SRV-ID,
URI-ID, or any application-specific identifier types supported by the
client.
Therefore, if and only if the presented identifiers do not include a
DNS-ID, SRV-ID, URI-ID, or any application-specific identifier types
supported by the client, then the client MAY as a last resort check
for a string whose form matches that of a fully qualified DNS domain
name in a Common Name field of the subject field (i.e., a CN-ID). If
the client chooses to compare a reference identifier of type CN-ID
against that string, it MUST follow the comparison rules for the DNS
domain name portion of an identifier of type DNS-ID, SRV-ID, or
URI-ID, as described under Section 6.4.1, Section 6.4.2, and
Section 6.4.3.
6.5. Matching the Application Service Type Portion
When a client checks identifiers of type SRV-ID and URI-ID, it MUST
check not only the DNS domain name portion of the identifier but also
the application service type portion. The client does this by
splitting the identifier into the DNS domain name portion and the
application service type portion (as described under Section 6.3),
then checking both the DNS domain name portion (as described under
Section 6.4) and the application service type portion as described in
the following subsections.
Implementation Note: An identifier of type SRV-ID or URI-ID
provides an application service type portion to be checked, but
that portion is combined only with the DNS domain name portion of
the SRV-ID or URI-ID itself. For example, if a client's list of

reference identifiers includes an SRV-ID of "_xmpp-
client.im.example.org" and a DNS-ID of "apps.example.net", the
client would check (a) the combination of an application service
type of "xmpp-client" and a DNS domain name of "im.example.org"
and (b) a DNS domain name of "apps.example.net". However, the
client would not check (c) the combination of an application
service type of "xmpp-client" and a DNS domain name of
"apps.example.net" because it does not have an SRV-ID of "_xmpp-
client.apps.example.net" in its list of reference identifiers.
6.5.1. SRV-ID
The application service name portion of an SRV-ID (e.g., "imaps")
MUST be matched in a case-insensitive manner, in accordance with
[DNS-SRV]. Note that the "_" character is prepended to the service
identifier in DNS SRV records and in SRV-IDs (per [SRVNAME]), and
thus does not need to be included in any comparison.
6.5.2. URI-ID
The scheme name portion of a URI-ID (e.g., "sip") MUST be matched in
a case-insensitive manner, in accordance with [URI]. Note that the
":" character is a separator between the scheme name and the rest of
the URI, and thus does not need to be included in any comparison.
6.6. Outcome
The outcome of the matching procedure is one of the following cases.
6.6.1. Case #1: Match Found
If the client has found a presented identifier that matches a
reference identifier, then the service identity check has succeeded.
In this case, the client MUST use the matched reference identifier as
the validated identity of the application service.
6.6.2. Case #2: No Match Found, Pinned Certificate
If the client does not find a presented identifier matching any of
the reference identifiers but the client has previously pinned the
application service's certificate to one of the reference identifiers
in the list it constructed for this communication attempt (as
"pinning" is explained under Section 1.8), and the presented
certificate matches the pinned certificate (including the context as
described under Section 7.1), then the service identity check has
succeeded.

6.6.3. Case #3: No Match Found, No Pinned Certificate
If the client does not find a presented identifier matching any of
the reference identifiers and the client has not previously pinned
the certificate to one of the reference identifiers in the list it
constructed for this communication attempt, then the client MUST
proceed as described under Section 6.6.4.
6.6.4. Fallback
If the client is an interactive client that is directly controlled by
a human user, then it SHOULD inform the user of the identity mismatch
and automatically terminate the communication attempt with a bad
certificate error; this behavior is preferable because it prevents
users from inadvertently bypassing security protections in hostile
situations.
Security Warning: Some interactive clients give advanced users the
option of proceeding with acceptance despite the identity
mismatch, thereby "pinning" the certificate to one of the
reference identifiers in the list constructed by the client for
this communication attempt. Although this behavior can be
appropriate in certain specialized circumstances, in general it
ought to be exposed only to advanced users. Even then it needs to
be handled with extreme caution, for example by first encouraging
even an advanced user to terminate the communication attempt and,
if the advanced user chooses to proceed anyway, by forcing the
user to view the entire certification path and only then allowing
the user to pin the certificate (on a temporary or permanent
basis, at the user's option).
Otherwise, if the client is an automated application not directly
controlled by a human user, then it SHOULD terminate the
communication attempt with a bad certificate error and log the error
appropriately. An automated application MAY provide a configuration
setting that disables this behavior, but MUST enable the behavior by
default.
7. Security Considerations
7.1. Pinned Certificates
As defined under Section 1.8, a certificate is said to be "pinned" to
a DNS domain name when a user has explicitly chosen to associate a
service's certificate with that DNS domain name despite the fact that
the certificate contains some other DNS domain name (e.g., the user
has explicitly approved "apps.example.net" as a domain associated
with a source domain of "example.com"). The cached name association

MUST take account of both the certificate presented and the context
in which it was accepted or configured (where the "context" includes
the chain of certificates from the presented certificate to the trust
anchor, the source domain, the application service type, the
service's derived domain and port number, and any other relevant
information provided by the user or associated by the client).
7.2. Wildcard Certificates
This document states that the wildcard character '*' SHOULD NOT be
included in presented identifiers but MAY be checked by application
clients (mainly for the sake of backward compatibility with deployed
infrastructure). As a result, the rules provided in this document
are more restrictive than the rules for many existing application
technologies (such as those excerpted under Appendix B). Several
security considerations justify tightening the rules:
o Wildcard certificates automatically vouch for any and all host
names within their domain. This can be convenient for
administrators but also poses the risk of vouching for rogue or
buggy hosts. See for example [Defeating-SSL] (beginning at slide
91) and [HTTPSbytes] (slides 38-40).
o Specifications for existing application technologies are not clear
or consistent about the allowable location of the wildcard
character, such as whether it can be:
* only the complete left-most label (e.g., *.example.com)
* some fragment of the left-most label (e.g., fo*.example.com,
f*o.example.com, or *oo.example.com)
* all or part of a label other than the left-most label (e.g.,
www.*.example.com or www.foo*.example.com)
* all or part of a label that identifies a so-called "public
suffix" (e.g., *.co.uk or *.com)
* included more than once in a given label (e.g.,
f*b*r.example.com
* included as all or part of more than one label (e.g.,
*.*.example.com)
These ambiguities might introduce exploitable differences in
identity checking behavior among client implementations and
necessitate overly complex and inefficient identity checking
algorithms.

o There is no specification that defines how the wildcard character
may be embedded within the A-labels or U-labels [IDNA-DEFS] of an
internationalized domain name [IDNA-PROTO]; as a result,
implementations are strongly discouraged from including or
attempting to check for the wildcard character embedded within the
A-labels or U-labels of an internationalized domain name (e.g.,
"xn--kcry6tjko*.example.org"). Note, however, that a presented
domain name identifier MAY contain the wildcard character as long
as that character occupies the entire left-most label position,
where all of the remaining labels are valid NR-LDH labels,
A-labels, or U-labels (e.g., "*.xn--kcry6tjko.example.org").
Notwithstanding the foregoing security considerations, specifications
that reuse this one can legitimately encourage continued support for
the wildcard character if they have good reasons to do so, such as
backward compatibility with deployed infrastructure (see, for
example, [EV-CERTS]).
7.3. Internationalized Domain Names
Allowing internationalized domain names can lead to the inclusion of
visually similar (so-called "confusable") characters in certificates;
for discussion, see for example [IDNA-DEFS].
7.4. Multiple Identifiers
A given application service might be addressed by multiple DNS domain
names for a variety of reasons, and a given deployment might service
multiple domains (e.g., in so-called "virtual hosting" environments).
In the default TLS handshake exchange, the client is not able to
indicate the DNS domain name with which it wants to communicate, and
the TLS server returns only one certificate for itself. Absent an
extension to TLS, a typical workaround used to facilitate mapping an
application service to multiple DNS domain names is to embed all of
the domain names into a single certificate.
A more recent approach, formally specified in [TLS-EXT], is for the
client to use the TLS "Server Name Indication" (SNI) extension when
sending the client_hello message, stipulating the DNS domain name it
desires or expects of the service. The service can then return the
appropriate certificate in its Certificate message, and that
certificate can represent a single DNS domain name.
To accommodate the workaround that was needed before the development
of the SNI extension, this specification allows multiple DNS-IDs,
SRV-IDs, or URI-IDs in a certificate; however, it explicitly
discourages multiple CN-IDs. Although it would be preferable to
forbid multiple CN-IDs entirely, there are several reasons at this